<IN>;    # skip "--start of file--"
while(  <IN>  ) {
    if(  $_ eq "#:lav\n"  ) {
        local $/= "#:eof\n";
        my $block= <IN>;
        # ... process $block here ...
    } elsif(  $_ eq /^--end of file--/  ) {
        warn "Unexpected line: $_";
    }
}
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Updated: Thanks to graff for noting that I wrote $\ when I meant $/.

                - tye